1. Field of the Invention
The present invention relates to a light exposure mask used in a photolithography step, and a method for manufacturing a semiconductor device which has a circuit including a thin film transistor (hereinafter referred to as a TFT) by using the light exposure mask. For example, the present invention relates to a method for manufacturing an electronic device which is mounted as a component with an electro-optical device typified by a liquid crystal display panel or a light emitting display device including an organic light emitting element.
Note that the term “semiconductor device” in this specification refers to a device in general that can operate by utilizing semiconductor characteristics, and an electro-optical device, a semiconductor circuit, and an electronic device are all included in the semiconductor device.
2. Description of the Related Art
In order to form a mask made of a photoresist in manufacturing a semiconductor device provided with a thin film transistor (hereinafter referred to as a TFT), many steps are required before and after the formation. The required steps are, for example, substrate cleaning, resist material application, pre-bake, light exposure, development, post-bake, and the like.
The mask made of a photoresist needs to be removed after etching treatment or doping treatment, and the removal also requires many steps. The required steps are, for example, ashing treatment using a gas selected from O2, H2O, CF4, and the like, peeling treatment using various kinds of chemicals, peeling treatment combining the ashing treatment and treatment using chemicals, and the like. In this case, the peeling treatment using chemicals requires steps such as chemical treatment, rinse treatment with pure water, and substrate drying. Therefore, the use of the mask made of a photoresist causes a problem of increasing the number of steps for manufacturing a semiconductor device. A reduction in the number of photolithography steps, that is, a reduction in the number of masks used in photolithography steps is needed for reduction in processing time and cost.
As a method for reducing the number of masks, a light exposure method is proposed which uses a light transmitting substrate that can transmit exposure light and a light exposure mask including a light shielding portion formed of chromium or the like over the light transmitting substrate and a semi-transmissive portion having a light intensity reduction function where lines of a light shielding material and spaces are repeatedly formed with respective predetermined line widths. A light exposure mask including a semi-transmissive portion formed by lines and spaces is also referred to as a gray-tone light exposure mask, and light exposure using this light exposure mask is also referred to as gray-tone light exposure. By using this light exposure mask, one photoresist layer can be developed to have at least two different thicknesses. Then, a layer to be etched is etched while ashing this photoresist, which enables two layers to be etched to be formed into different patterns through a single photolithography step. This can eliminate one photolithography step, that is, one mask (for example, see Reference 1: Japanese Patent Laid-Open No. 2002-151523).
In the case of using a light exposure apparatus with a projection magnification of 1:1, for example, this light exposure mask including a semi-transmissive portion is formed so that the width of each of the lines and the spaces is smaller than the resolution (resolution limit) of the light exposure apparatus. Accordingly, the lines and spaces are not resolved but blurred over the substrate; therefore, the amount of exposure light transmitted through the semi-transmissive portion over the substrate is almost homogenized over almost the entire semi-transmissive portion. As a result, the photoresist layer can be formed with small thickness in the semi-transmissive portion.
However, in the case of using a light exposure apparatus with high resolution, there is a case where the photoresist layer cannot be formed with small thickness even when each of lines and spaces is formed smaller than the resolution or cannot be formed with uniform thickness even when the photoresist layer can be formed with small thickness. FIGS. 13A and 13B show an example thereof. FIG. 13A shows a schematic cross-sectional view of a light exposure mask, and FIG. 13B shows a cross-sectional photograph of a photoresist layer obtained by light exposure and development using the light exposure mask. The position of the light exposure mask in FIG. 13A and the position of the photoresist layer in the cross-sectional photograph of FIG. 13B practically correspond to each other. A light exposure apparatus with a projection magnification of 1:1 and a resolution of 1.5 μm is used. Even when using a light exposure mask of which each of lines and spaces is smaller than the resolution as shown in FIG. 13A, there is a case where a photoresist layer having a region with small and uniform thickness cannot be formed. In a region of the light exposure mask of FIG. 13A where each line has a width of 1.0 μm and each space has a width of 0.5 μm, the photoresist layer is not formed with small thickness but with large thickness, almost the same thickness as that of a light shielding portion. Furthermore, when a semi-transmissive portion is provided on a side of a light shielding portion with both a line width and a space width of 0.5 μm, there is a case where the photoresist layer cannot be formed with small thickness and is formed into a tapered shape in which a thickness is decreased from a light shielding portion toward a transmissive portion.